The present invention relates to a tool for coldforming and drawing operations, particularly steel tire cord drawing operations.
The performance of a drawing die in production of steel tire cord is improved by increasing the hardness of the cemented carbide. Coarse wire is usually dry drawn by grades with 10 wt-% or 6 wt-% Co and a hardness 1600 and 1750 Vickers respectively. Wet drawing from 1.5-2 mm down to final dimension, 0.15-0.3 mm, is usually made with drawing dies in grades having a hardness of from about 1900-2000 HV and Co binder content <5 wt-%, most often around 3 wt-%.
In the 1980's a grade having only 3 wt-% binder and ultra fine grain size for tire cord drawing was introduced by Sandvik. It was later withdrawn due to the low strength and brittle behaviour leading to premature failures.
In a European project, Wireman, (reported by A. M. Massai et al, “Scientific and technological progress in the field of steel wire drawing”, Wire 6/1999), the conditions for drawing of tire cord were investigated. New cemented carbide grades were tested in the grain size range of 0.3-1 μm and a binder content of 0.3-5 wt-%. A hardness increase was achieved by reducing the binder content and decreasing the grain size of WC. According to published results, the grades did not completely satisfy the expectation on better performance, despite the high hardness achieved. The conclusion quotes: “The wear tests demonstrated that not only the hardness of the dies controls the die wear mechanism.”
According to U.S. Pat. No. 6,464,748, beside hardness of cemented carbide, corrosion is a major factor controlling the wear resistance. Normally higher Co binder content leads to higher sensitivity to corrosion and said US-patent discloses improvements by low binder content and alloying of the cobalt binder with nickel and chromium to make it corrosion resistant, i.e. a similar approach as in the above mentioned Wireman project.
U.S. Pat. No. 5,948,523 discloses a coldforming tool with an improved hard wearing surface zone. This has been achieved by a post-sintering heat treatment in a boron nitride containing environment of a hard metal of a suitable composition. The effect is most pronounced when the heat treatment is made of a hard metal which has previously been sintered to achieve a high carbon content through a suitable choice of chemical composition and processing conditions.
During many years there has been an ongoing development of cemented carbide with finer and finer grain size.
The extension of cemented carbide grain sizes into the ultra fine size range leads to a number of positive improvements regarding the wear processes.
Attrition wear (or grain loss volume) may be reduced by an order of magnitude by little more than halving the sintered grain size (in the absence of other wear processes), since grain volume is related to the cube of diameter.
Adhesive fracture is another dangerous kind of attrition wear, in which the separation of strongly welded tool-workmaterial interfaces can induce tensile cleavage within the underlying carbide. Ultra fine hardmetals can resist the onset of such fractures better than coarser ones due to their greater rupture strength.
Erosion/corrosion of the binder phase is said to be part of the wear mechanism in wire drawing. Even though the content of binder is increased in ultra fine cemented carbide the smaller WC grain size leads to thinner binder films, generally called binder free path. Thus resistance to selective erosion of the soft binderphase by wear particles is reduced. It is reasonable to believe that the thinner binder also leads to better oxidation/corrosion properties since the properties of the binder at the WC interface is different from the pure metal.
From the above it seems that the main interest in developing finer sub-micron hardmetal, perhaps into the nanometer range, is to raise hardness, maximise attrition wear resistance and strength whilst as far as possible maintaining all other attributes at useful levels.
It has now been found that use of ultra fine grained cemented carbide with a Co content >5 wt-% can lead to improved performance in steel tire cord production by the combination of the improvements in strength, hardness and toughness of ultra fine cemented carbide.